Many operations carried out in refineries and petrochemical plants involve feeding a hydrocarbon/hydrogen stream to a reactor, withdrawing a reactor effluent stream of different hydrocarbon/hydrogen composition, separating the effluent into liquid and vapor portions, and recirculating part of the vapor stream to the reactor, so as to reuse unreacted hydrogen. Such loop operations are found, for example, in the hydrotreater, hydrocracker and catalytic reformer sections of most modern refineries, as well as in isomerization reactors and hydrodealkylation units.
The phase separation into liquid and vapor portions is often carried out in one or more steps by simply changing the pressure and/or temperature of the effluent. Therefore, in addition to hydrogen, the overhead vapor from the phase separation usually contains light hydrocarbons, particularly methane and ethane, and various contaminants, such as hydrogen sulfide, carbon dioxide, and ammonia. In a closed recycle loop, these components build up, change the reactor equilibrium conditions and can lead to reduced product yield and premature deactivation of reactor catalysts. This build-up of undesirable contaminants is usually controlled by purging a part of the vapor stream from the loop. Such a purge operation is unselective however, and, since the purge stream may contain as much as 80 vol % or more hydrogen, multiple volumes of hydrogen can be lost from the loop for every volume of contaminant that is purged. The purge stream may be treated by further separation in some downstream operation, or may simply pass to the plant fuel header.
The impetus for hydrogen recovery in the reactor loop is two-fold. First, demand for hydrogen in refineries and petrochemical plants is high, and it is almost always more cost-effective to try to reuse as much gas as is practically possible than to meet the hydrogen demand entirely from fresh stocks. Secondly, it is desirable in most operations to maintain a high hydrogen partial pressure in the reactor. The availability of ample hydrogen during the reaction step prolongs the life of the catalyst by controlling coke formation, and suppresses the formation of non-preferred, low value products. Furthermore, many streams also contain high percentages, such as 10%, 20%, 30% or more, of C.sub.3+ hydrocarbons. The chemical value of these individual components is much higher--in some instances, as much as eight times higher--than their fuel value. The ability to recover at least some of this value would be advantageous, especially in refineries, which generally operate at narrow financial margins.
Hydrogen recovery techniques that have been deployed in refineries include, besides simple phase separation of fluids, pressure swing adsorption (PSA) and membrane separation. U.S. Pat. No. 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. The membrane permeates the hydrogen selectively and produces a hydrogen-enriched gas product that is withdrawn from the process. U.S. Pat. No. 5,053,067, to L'Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to change the dewpoint of the gas to facilitate downstream treatment. U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, shows treatment of light ends containing hydrogen and light hydrocarbons, including using a hydrogen-selective membrane to separate hydrogen from other components. U.S. Pat. No. 5,689,032, to Krause/Pasadyn, discusses a method for separating hydrogen and hydrocarbons from refinery off-gases, including multiple low-temperature condensation steps and a membrane separation step for hydrogen removal.
A chapter in "Polymeric Gas Separation Membranes", D. R. Paul et al. (Eds.) entitled "Commercial and Practical Aspects of Gas Separation Membranes", by Jay Henis describes various hydrogen separations that can be performed with hydrogen-selective membranes.
Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
U.S. Pat. No. 4,857,078, to Watler, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane. Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. No. 4,654,063 and U.S. Pat. No. 4,836,833, to Air Products, and U.S. Pat. No. 4,892,564, to Cooley.
U.S. Pat. No. 5,332,424, to Air Products, describes fractionation of a gas stream containing light hydrocarbons and hydrogen using an "adsorbent membrane". The membrane is made of carbon, and selectively adsorbs hydrocarbons onto the carbon surface, allowing separation between various hydrocarbon fractions to be made. Hydrogen tends to be retained in the membrane residue stream. Other Air Products patents that show application of carbon adsorbent membranes to hydrogen/hydrocarbon separations include U.S. Pat. Nos. 5,354,547; 5,435,836; 5,447,559 and 5,507,856, which all relate to purification of streams from steam reformers. U.S. Pat. No. 5,634,354, to Air Products, discloses removal of hydrogen from hydrogen/olefin streams. In this case, the membrane used to perform the separation is either a polymeric membrane selective for hydrogen over hydrocarbons or a carbon adsorbent membrane selective for hydrocarbons over hydrogen. U.S. Pat. No. 5,082,481, to Lummus Crest, describes removal of carbon dioxide, hydrogen and water vapor from cracking effluent, the hydrogen separation being accomplished by a hydrogen-selective membrane.
The use of certain polymeric membranes to treat off-gas streams in refineries is also described in the following papers: "Hydrogen Purification with Cellulose Acetate Membranes", by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; "Prism.TM. Separators Optimize Hydrocracker Hydrogen", by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; "Plant Uses Membrane Separation", by H. Yamashiro et al., in Hydrocarbon Processing, February 1985; and "Optimizing Hydrocracker Hydrogen", by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. These papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons. The use of membranes in refinery separations is also mentioned in "Hydrogen Technologies to Meet Refiners' Future Needs", by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. This paper points out the disadvantage of membranes, namely that they permeate the hydrogen, thereby delivering it at low pressure, and that they are susceptible to damage by hydrogen sulfide and heavy hydrocarbons.
U.S. Pat. No. 4,362,613, to Monsanto, describes a process for treating the vapor phase from a high pressure separator in a hydrocracking plant by passing the vapor across a membrane that is selectively permeable to hydrogen. The process yields a hydrogen-enriched permeate that can be recompressed and recirculated to the hydrocracker reactor. U.S. Pat. No. 4,367,135, also to Monsanto, describes a process in which effluent from a low pressure separator is treated to recover hydrogen using the same type of hydrogen-selective membrane. Because these membranes permeate the hydrogen to the low pressure side of the membrane, the permeate stream must be recompressed before being reintroduced to the hydroprocessing reactor. In addition, these types of membranes do not display good resistance to damage by water vapor or acid gases that are often present in the effluent streams.
U.S. Pat. No. 4,980,046, to UOP, discusses desulfurization of a hydroprocessor effluent by flash evaporation and/or adsorption.